| Summary: | The sinus venosus, the cardiac chamber upstream of the (right) atrium, is a severely underinvestigated structure. Yet, its myocardium harbors the cardiac pacemaker in all vertebrates. In human, ectopic pacemaking and subsequent pathologies may originate from sinus venosus-derived myocardium surrounding the coronary sinus and the superior caval vein. In ectothermic vertebrates, i.e., fishes, amphibians and reptiles, the sinus venosus aids atrial filling by contracting prior to the atrium (atria). This is facilitated by the sinuatrial delay of approximately the same duration as the atrioventricular delay, which facilitates atrial filling of the ventricles. In mammals, the sinuatrial delay is lost, and the sinus venosus-derived myocardium persists as an extensive myocardial sheet surrounding the caval veins, which is activated in synchrony with the myocardium of the atria. The caval vein myocardium is hardly of significance in the healthy formed heart, but we suggest that the sinus venosus functions as a chamber during development when cardiac output, heart rate, blood pressure and architecture is much more like that of ectothermic vertebrates. The remodeling of the sinus venosus in mammals may be an adaptation associated with the high heart rates necessary for postnatal endothermy. If so, the endothermic birds should exhibit a similar remodeling as mammals, which remains to be investigated.
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